It's not unusual for Apple to take a long time fixing updates, but this update is special. One of the vulnerabilities addressed is CVE-2003-0063 in the X11 component, a vulnerability disclosed in February of 2003. Another flaw in the iChat Server, goes back to 2006. 9 more date back to 2008 and 30 to 2009. 27 of the issues are described as being capable of resulting in remote code execution.

This update will take a while. The Snow Leopard versions range from 719MB to 897MB. The Leopard client version weighs in at a slim 78MB.